1. Field of the Invention
The present invention relates to a method of data communication in a communication network on an automobile, and more particularly to a method of data communication in a network of components of an audiovisual system on an automobile.
2. Description of the Prior Art
In recent years, automobile audio systems have been changing from systems which give a user only audio information such as music to systems which allow the user to enjoy both audio and visual information. Systems with both audio and visual capabilities are known as audiovisual systems.
Automobile audio systems on automobiles are composed of many different audio and visual components. The audio components include a cassette tape deck, a radio tuner, a CD (compact disk) player, etc., while the visual components include a TV (television) tuner, a navigation system, etc. Audio signals produced by some of these components are amplified by an amplifier and reproduced from loudspeakers in the automobile. Video signals produced by other components are displayed on a display unit in the automobile. Today, the audiovisual system components are controlled according to digital technology, i.e., by respective controllers in the form of microcomputers.
The components of an audiovisual system are required to be controlled systematically for systematic operation of the components. To meet this requirement, the controllers of the respective components are connected by a communication bus network, so that control data for the components will be transmitted through the communication bus.
In the conventional audiovisual system network, the controllers of the components are controlled by a polling process. According to the polling process, either one of the controllers is given a priority status and handled as a master unit, with the other controllers as slave units, and when the master unit is to collect data from a slave unit, the master unit always accesses the slave unit, i.e., initiates the transfer of the data from the slave unit.
When the master unit transmits communication data to a slave unit for access or the slave unit transmits data back to the master unit, it is necessary that the controllers be identified or specified. For this purpose, the controllers are allotted respective address indicating themselves.
Heretofore, the controllers are allotted respective unique addresses. Control data which are to be given to controllers comprise address data peculiar to the respective controllers and instruction data (e.g., start command: ON data) added to the address data. The control data which are generated are sent to the communication bus.
In networks with communication buses, one master unit is necessarily connected to a communication bus. If the master unit fails for some reason, then it can no longer transmit a polling signal to the communication bus. In the event of the trouble, the slave units regard the master unit as failing upon detection of no polling signal within a predetermined period of time, disable themselves, enter a low power consumption mode, and stand by in that mode. When the master unit recovers from the failure or a new master unit is connected to the communication bus, the master unit sends a polling signal to access the slave units which have been standing by in the low power consumption mode.
Each time an ACC switch (car accessory power supply switch) assembled together with an engine start keyswitch on an automobile is turned on or off, the audiovisual system on the automobile is also turned on or off. Therefore, the connection status of the slave units of the network has to be confirmed each time the ACC switch is turned on. According to the polling process, each time the ACC switch is turned on, the master unit has to access the individual slave units for confirming their connection, and hence is placed under a very large burden. For example, if each of the slave units has a 12-bit address, then the master unit has to access the slave units 4096 times at maximum provided the master unit confirms the connection of the slave units to the communication bus through its serial port.
In order for the master unit to access the slave units according to the polling process, it is necessary for the master unit to register the addresses of all the slave units connected to the communication bus. Any additional slave unit whose address has not been registered by the master unit cannot be accessed by the master unit even if the additional slave unit is physically or electrically connected to the communication bus. Therefore, to access the additional slave unit reliably, the additional slave unit should be allotted its own unique address, and that address should be newly registered by the master unit.
One solution would be for the slave units to access the master unit to request for the confirmation of a connection through a self report, The master unit would establish times to receive access signals from the slave units. Since, however, the slave units are enabled at different times, a slave unit which was connected when confirmed previously and which is to be connected this time might not be connected and might possibly be disconnected because the times to receive access signals from the slave units are fixed.
In the above system, when the master unit fails for some reason and then recovers from the malfunction, the system may not recover automatically. If the malfunctioning condition of the master unit continues, then the slave units consume electric energy wastefully.